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Register a new userTracing the assembly of the Milky Ways disk through abundance clustering
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Authors
Bridget L. Ratcliffe
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A major goal in the field of galaxy formation is to understand the formation of the Milky Ways disk. The first step toward doing this is to empirically describe its present state. We use the new high-dimensional dataset of 19 abundances from 27,135 red clump APOGEE stars to examine the distribution of clusters defined using abundances. We explore different dimensionality reduction techniques and implement a non-parametric agglomerate hierarchical clustering method. We see that groups defined using abundances are spatially separated, as a function of age. Furthermore, the abundance groups represent different distributions in the [Fe/H]-age plane. Ordering our clusters by age reveals patterns suggestive of the sequence of chemical enrichment in the disk over time. Our results indicate that a promising avenue to trace the details of the disks assembly is via a full interpretation of the empirical connections we report.
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We employ the first two years of data from the near-infrared, high-resolution SDSS-III/APOGEE spectroscopic survey to investigate the distribution of metallicity and alpha-element abundances of stars over a large part of the Milky Way disk. Using a sample of ~10,000 kinematically-unbiased red-clump stars with ~5% distance accuracy as tracers, the [alpha/Fe] vs. [Fe/H] distribution of this sample exhibits a bimodality in [alpha/Fe] at intermediate metallicities, -0.9<[Fe/H]<-0.2, but at higher metallicities ([Fe/H]=+0.2) the two sequences smoothly merge. We investigate the effects of the APOGEE selection function and volume filling fraction and find that these have little qualitative impact on the alpha-element abundance patterns. The described abundance pattern is found throughout the range 5<R<11 kpc and 0<|Z|<2 kpc across the Galaxy. The [alpha/Fe] trend of the high-alpha sequence is surprisingly constant throughout the Galaxy, with little variation from region to region (~10%). Using simple galactic chemical evolution models we derive an average star formation efficiency (SFE) in the high-alpha sequence of ~4.5E-10 1/yr, which is quite close to the nearly-constant value found in molecular-gas-dominated regions of nearby spirals. This result suggests that the early evolution of the Milky Way disk was characterized by stars that shared a similar star formation history and were formed in a well-mixed, turbulent, and molecular-dominated ISM with a gas consumption timescale (1/SFE) of ~2 Gyr. Finally, while the two alpha-element sequences in the inner Galaxy can be explained by a single chemical evolutionary track this cannot hold in the outer Galaxy, requiring instead a mix of two or more populations with distinct enrichment histories.
We study the evolution of oxygen abundance radial gradients as a function of time for the Milky Way Galaxy obtained with our {sc Mulchem} chemical evolution model. We review the recent data of abundances for different objects observed in our Galactic disc. We analyse with our models the role of the growth of the stellar disc, as well as the effect of infall rate and star formation prescriptions, or the pre-enrichment of the infall gas, on the time evolution of the oxygen abundance radial distribution. We compute the radial gradient of abundances within the {sl disk}, and its corresponding evolution, taking into account the disk growth along time. We compare our predictions with the data compilation, showing a good agreement. Our models predict a very smooth evolution when the radial gradient is measured within the optical disc with a slight flattening of the gradient from $sim -0.057$,dex,kpc$^{-1}$ at $z=4$ until values around $sim -0.015$,dex,kpc$^{-1}$ at $z=1$ and basically the same gradient until the present, with small differences between models. Moreover, some models show a steepening at the last times, from $z=1$ until $z=0$ in agreement with data which give a variation of the gradient in a range from $-0.02$ to $-0.04$,de,kpc$^{-1}$ from $t=10$,Gyr until now. The gradient measured as a function of the normalized radius $R/R_{rm eff}$ is in good agreement with findings by CALIFA and MUSE, and its evolution with redshift falls within the error bars of cosmological simulations.
MeerKAT radio continuum and XMM-Newton X-ray images have recently revealed a spectacular bipolar channel at the Galactic Center that spans several degrees ($sim$0.5 kpc). An intermittent jet likely formed this channel and is consistent with earlier evidence of a sustained, Seyfert-level outburst fueled by black-hole accretion onto SgA* several Myr ago. Therefore, to trace an intermittent jet that perhaps penetrated, deflected, and percolated along multiple paths through the interstellar medium, relevant interactions are identified and quantified in archival X-ray images, Hubble Space Telescope Paschen-$alpha$ images and ALMA mm-wave spectra, and new SOAR telescope IR spectra. Hydrodynamical simulations are used to show how a currently weak jet can explain these structures and inflate the ROSAT/eROSITA X-ray and Fermi $gamma$-ray bubbles that extend $pm$60 deg from the Galactic plane. Thus, our Galactic outflow has features in common with energetic, jet-driven structures in the prototypical Seyfert galaxy NGC 1068.
I present the mean metallicity distribution of stars in the Milky Way Galaxy based on photometry from the Sloan Digital Sky Survey. I utilize an empirically calibrated set of stellar isochrones developed in previous work to estimate the metallicities of individual stars to a precision of $0.2$ dex for reasonably bright stars across the survey area. I also obtain more precise metallicity estimates using priors from the $Gaia$ parallaxes for relatively nearby stars. Close to the Galactic mid-plane ($|Z|<2$ kpc), a mean metallicity map reveals deviations from the mirror symmetry between the northern and southern hemispheres, displaying wave-like oscillations. The observed metallicity asymmetry structure is almost parallel to the Galactic mid-plane, and coincides with the previously known asymmetry in the stellar number density distribution. This result reinforces the previous notion of the plane-parallel vertical waves propagating through the disk, in which a local metallicity perturbation from the mean vertical metallicity gradient is induced by the phase-space wrapping of stars in the $Z$-$V_Z$ plane. The maximum amplitude of the metallicity asymmetry ($Delta$[Fe/H]$sim0.05$) implies that these stars have been pulled away from the Galactic mid-plane by an order of $Delta|Z|sim80$ pc as a massive halo substructure such as the Sagittarius dwarf galaxy plunged through the Milky Way. This work provides evidence that the $Gaia$ phase-space spiral may continue out to $|Z|sim1.5$ kpc.
We analyse the chemical properties of a set of solar vicinity stars, and show that the small dispersion in abundances of alpha-elements at all ages provides evidence that the SFH has been uniform throughout the thick disk. In the context of long time scale infall models, we suggest that this result points either to a limited dependence of the gas accretion on the Galactic radius in the inner disk (R<10 kpc), or to a decoupling of the accretion history and star formation history due to other processes governing the ISM in the early disk, suggesting that infall cannot be a determining parameter of the chemical evolution at these epochs. We argue however that these results and other recent observational constraints -- namely the lack of radial metallicity gradient and the non-evolving scale length of the thick disk -- are better explained if the early disk is viewed as a pre-assembled gaseous system, with most of the gas settled before significant star formation took place -- formally the equivalent of a closed-box model. In any case, these results point to a weak, or non-existent inside-out formation history in the thick disk, or in the first 3-5 Gyr of the formation of the Galaxy. We argue however that the growing importance of an external disk whose chemical properties are distinct from those of the inner disk would give the impression of an inside-out growth process when seen through snapshots at different epochs. However, the progressive, continuous process usually invoked may not have actually existed in the Milky Way.
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